Initializing and testing integrated circuits with selectable scan chains with exclusive-or outputs

ABSTRACT

Embodiments of the invention provide a scan test system for an integrated circuit comprising multiple processing elements. The system comprises at least one scan input component and at least one scan clock component. Each scan input component is configured to provide a scan input to at least two processing elements. Each scan clock component is configured to provide a scan clock signal to at least two processing elements. The system further comprises at least one scan select component for selectively enabling a scan of at least one processing element. Each processing element is configured to scan in a scan input and scan out a scan output when said the processing element is scan-enabled. The system further comprises an exclusive-OR tree comprising multiple exclusive-OR logic gates. The said exclusive-OR tree generates a parity value representing a parity of all scan outputs scanned out from all scan-enabled processing elements.

This invention was made with Government support under HR0011-09-C-0002awarded by Defense Advanced Research Projects Agency (DARPA). TheGovernment has certain rights in this invention.

BACKGROUND

Embodiments of the invention relate to integrated circuits, and inparticular, initializing and testing integrated circuits.

An integrated circuit comprises a set of electronic circuits disposed ona semiconductor wafer or substrate. The set of electronic circuits mayinclude multiple processing elements. There are different types ofprocessing elements, such as microprocessors, microcontrollers, digitalsignal processors, graphics processors, reconfigurable processors, fixedfunction units, hardware accelerators, neurosynaptic neural corecircuits, etc. The processing elements may be arranged in aone-dimensional grid arrangement, a two-dimensional grid arrangement, athree-dimensional grid arrangement, or in a ring or torus topology. Theprocessing elements may be interconnected, thereby enabling packetcommunication between the processing elements.

Manufacturing testing of a semiconductor integrated circuit is anessential part of the production of the processing elements. Typically,manufacturing testing is carried out using a scanning methodology thatscans in test data into an integrated circuit with a scan chain. A scanchain may comprise long shift registers. A test is then run by drivingthe integrated circuit using the scanned in test data, and collectingtest results for the integrated circuit. The test results are scannedout of the scan chain.

The bigger/larger the size of an integrated circuit, the longer its scanchain. As such, it takes a proportionally longer time to scan in testdata and scan out test results for a bigger/larger-sized integratedcircuit, thereby increasing the time for testing the integrated circuitand increasing the cost of production. A number of compression schemesand built-in-test circuits are available to mitigate this problem.However, a built-in-test circuit consumes area and power, and increasesthe complexity of the integrated circuit.

For example, in a processing system with multiple units on a chip (e.g.,many-core processors, neuromorphic processors, GPU, and FPGA chips),implementing a built-in-test circuit for each unit becomes costprohibitive. Further, implementing a centralized built-in-test circuitfor the processing system may not easily resolve the problems ofincreased time for testing and increased complexity of the integratedcircuit.

Further, if a scan chain is also used to initialize an integratedcircuit, a longer scan chain may lead to slow bring-up time of a digitalsystem implemented using the integrated circuit. Fast scan chainingsystem are needed for speedy initialization. For example, in anintegrated circuit that may not have a high speed clock (e.g., aneuromorphic circuit), fast initialization of the integrated circuitusing a slow clock is essential.

BRIEF SUMMARY

In one embodiment, a scan test system for an integrated circuitcomprising multiple processing elements is provided. The systemcomprises at least one scan input component and at least one scan clockcomponent. Each scan input component is configured to provide a scaninput to at least two processing elements. Each scan clock component isconfigured to provide a scan clock signal to at least two processingelements. The system further comprises at least one scan selectcomponent for selectively enabling a scan of at least one processingelement. Each processing element is configured to scan in a scan inputand scan out a scan output when said the processing element isscan-enabled. The system further comprises an exclusive-OR treecomprising multiple exclusive-OR logic gates. The said exclusive-OR treegenerates a parity value representing a parity of all scan outputsscanned out from all scan-enabled processing elements.

Another embodiment provides a method for initializing and testing anintegrated circuit comprising multiple processing elements. The methodcomprises providing a scan input to at least two of the multipleprocessing elements, and selectively enabling a scan of at least one ofthe multiple processing elements. Each processing element is configuredto scan in a scan input and scan out a scan output when the processingelement is scan-enabled. The method further comprises generating aparity value representing a parity of all scan outputs scanned out fromall scan-enabled processing elements using an exclusive-OR treecomprising multiple exclusive-OR logic gates.

These and other features, aspects and advantages of the presentinvention will become understood with reference to the followingdescription, appended claims and accompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A illustrates an example neurosynaptic core circuit (“corecircuit”), in accordance with an embodiment of the invention;

FIG. 1B illustrates a block diagram of an example integrated circuit(“chip circuit”), in accordance with an embodiment of the invention;

FIG. 2 illustrates an example scan system for a chip circuit, inaccordance with an embodiment of the invention;

FIG. 3 illustrates an example triangulation process to identify a singlefailed unit of a chip circuit, in accordance with an embodiment of theinvention;

FIG. 4 illustrates an example scan loop system for a unit, in accordancewith an embodiment of the invention;

FIG. 5 illustrates another example scan system for a chip circuit, inaccordance with an embodiment of the invention;

FIG. 6 illustrates a flowchart of an example process for scanning a chipcircuit in a parallel scan mode, in accordance with an embodiment of theinvention;

FIG. 7 illustrates a flowchart of an example process for triangulating afailed unit of a chip circuit using a parallel scan mode and a delayedscan mode, in accordance with an embodiment of the invention;

FIG. 8 illustrates a flowchart of an example process for triangulating afailed unit of a chip circuit by combining a binary search with aparallel scan mode and a delayed scan mode, in accordance with anembodiment of the invention;

FIG. 9 illustrates a flowchart of an example process for scanning eachunit of a chip circuit using an individual scan mode, in accordance withan embodiment of the invention;

FIG. 10 illustrates a flowchart of an example process for triangulatingone or more failed units of a chip circuit using a parallel scan modeand a multiple delayed scan mode with multiple delays, in accordancewith an embodiment of the invention; and

FIG. 11 is a high level block diagram showing an information processingsystem useful for implementing one embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to integrated circuits, and inparticular, initializing and testing integrated circuits. One embodimentprovides a scan test system for an integrated circuit comprisingmultiple processing elements. The scan test system allows for scan inputto be provided to at least two processing elements in parallel. The scantest system can selectively enable a scan of at least one processingelement based on scan input provided. Each scan-enabled processingelement scans in scan input and scans out a resulting scan output. Scanoutputs scanned out from scan-enabled processing elements may becompared against expected test results to determine whether testing ofthe circuit is successful.

In one embodiment, a neurosynaptic system comprises a system thatimplements neuron models, synaptic models, neural algorithms, and/orsynaptic algorithms. In one embodiment, a neurosynaptic system comprisessoftware components and/or hardware components, such as digitalhardware, analog hardware or a combination of analog and digitalhardware (i.e., mixed-mode).

The term electronic neuron as used herein represents an architectureconfigured to simulate a biological neuron. An electronic neuron createsconnections between processing elements that are roughly functionallyequivalent to neurons of a biological brain. As such, a neuromorphic andsynaptronic computation comprising electronic neurons according toembodiments of the invention may include various electronic circuitsthat are modeled on biological neurons. Further, a neuromorphic andsynaptronic computation comprising electronic neurons according toembodiments of the invention may include various processing elements(including computer simulations) that are modeled on biological neurons.Although certain illustrative embodiments of the invention are describedherein using electronic neurons comprising electronic circuits, thepresent invention is not limited to electronic circuits. A neuromorphicand synaptronic computation according to embodiments of the inventioncan be implemented as a neuromorphic and synaptronic architecturecomprising circuitry, and additionally as a computer simulation. Indeed,embodiments of the invention can take the form of an entirely hardwareembodiment, an entirely software embodiment or an embodiment containingboth hardware and software elements.

The term electronic axon as used herein represents an architectureconfigured to simulate a biological axon that transmits information fromone biological neuron to different biological neurons. In oneembodiment, an electronic axon comprises a circuit architecture. Anelectronic axon is functionally equivalent to axons of a biologicalbrain. As such, neuromorphic and synaptronic computation involvingelectronic axons according to embodiments of the invention may includevarious electronic circuits that are modeled on biological axons.Although certain illustrative embodiments of the invention are describedherein using electronic axons comprising electronic circuits, thepresent invention is not limited to electronic circuits.

FIG. 1A illustrates an example neurosynaptic core circuit (“corecircuit”) 10, in accordance with an embodiment of the invention. Thecore circuit 10 comprises a plurality of electronic neurons (“neurons”)11 and a plurality of electronic axons (“axons”) 15. The neurons 11 andthe axons 15 are interconnected via an m x n crossbar 12 comprisingmultiple intra-core electronic synapse devices (“synapses”) 31, multiplerows/axon paths 26, and multiple columns/dendrite paths 34, wherein “x”represents multiplication, and m and n are positive integers.

Each synapse 31 communicates firing events (e.g., spike events) betweenan axon 15 and a neuron 11. Specifically, each synapse 31 is located atcross-point junction between an axon path 26 and a dendrite path 34,such that a connection between the axon path 26 and the dendrite path 34is made through the synapse 31. Each axon 15 is connected to an axonpath 26, and sends firing events to the connected axon path 26. Eachneuron 11 is connected to a dendrite path 34, and receives firing eventsfrom the connected dendrite path 34. Therefore, each synapse 31interconnects an axon 15 to a neuron 11, wherein, with respect to thesynapse 31, the axon 15 and the neuron 11 represent an axon of apre-synaptic neuron and a dendrite of a post-synaptic neuron,respectively.

Each synapse 31 and each neuron 11 has configurable operationalparameters. In one embodiment, the core circuit 10 is a uni-directionalcore, wherein the neurons 11 and the axons 15 of the core circuit 10 arearranged as a single neuron array and a single axon array, respectively.In another embodiment, the core circuit 10 is a bi-directional core,wherein the neurons 11 and the axons 15 of the core circuit 10 arearranged as two neuron arrays and two axon arrays, respectively. Forexample, a bi-directional core circuit 10 may have a horizontal neuronarray, a vertical neuron array, a horizontal axon array and a verticalaxon array, wherein the crossbar 12 interconnects the horizontal neuronarray and the vertical neuron array with the vertical axon array and thehorizontal axon array, respectively.

In response to the firing events received, each neuron 11 generates afiring event according to a neuronal activation function. A preferredembodiment for the neuronal activation function can be leakyintegrate-and-fire.

An external two-way communication environment may supply sensory inputsand consume motor outputs. The neurons 11 and axons 15 are implementedusing complementary metal-oxide semiconductor (CMOS) logic gates thatreceive firing events and generate a firing event according to theneuronal activation function. In one embodiment, the neurons 11 andaxons 15 include comparator circuits that generate firing eventsaccording to the neuronal activation function. In one embodiment, thesynapses 31 are implemented using 1-bit static random-access memory(SRAM) cells. Neurons 11 that generate a firing event are selected oneat a time, and the firing events are delivered to target axons 15,wherein the target axons 15 may reside in the same core circuit 10 orsomewhere else in a larger system with many core circuits 10.

As shown in FIG. 1A, the core circuit 10 further comprises anaddress-event receiver (Core-to-Axon) 4, an address-event transmitter(Neuron-to-Core) 5, and a controller 6 that functions as a global statemachine (GSM). The address-event receiver 4 receives firing events andtransmits them to target axons 15. The address-event transmitter 5transmits firing events generated by the neurons 11 to the core circuits10 including the target axons 15.

The controller 6 sequences event activity within a time-step. Thecontroller 6 divides each time-step into operational phases in the corecircuit 10 for neuron updates, etc. In one embodiment, within atime-step, multiple neuron updates and synapse updates are sequentiallyhandled in a read phase and a write phase, respectively. Further,variable time-steps may be utilized wherein the start of a nexttime-step may be triggered using handshaking signals whenever theneuron/synapse operation of the previous time-step is completed. Forexternal communication, pipelining may be utilized wherein load inputs,neuron/synapse operation, and send outputs are pipelined (thiseffectively hides the input/output operating latency).

As shown in FIG. 1A, the core circuit 10 further comprises a routingfabric 70. The routing fabric 70 is configured to selectively routeneuronal firing events among core circuits 10. The routing fabric 70comprises a firing events address lookup table (LUT) module 57, a packetbuilder (PB) module 58, a head delete (HD) module 53, and a core-to-corepacket switch (PSw) 55. The LUT 57 is an N address routing table isconfigured to determine target axons 15 for firing events generated bythe neurons 11 in the core circuit 10. The target axons 15 may be axons15 in the same core circuit 10 or other core circuits 10. The LUT 57retrieves information such as target distance, direction, addresses, anddelivery times (e.g., about 19 bits/packet×4 packets/neuron). The LUT 57converts firing events generated by the neurons 11 into forwardingaddresses of the target axons 15.

The PB 58 packetizes the routing information retrieved by the LUT 57into outgoing address-event packets. The core-to-core PSw 55 is anup-down-left-right mesh router configured to direct the outgoingaddress-event packets to the core circuits 10 containing the targetaxons 15. The core-to-core PSw 55 is also configured to receive incomingaddress-event packets from the core circuits 10. The HD 53 removesrouting information from an incoming address-event packet to deliver itas a time stamped firing event to the address-event receiver 4.

In one example implementation, the core circuit 10 may comprise 256neurons 11. The crossbar 12 may be a 256×256 ultra-dense crossbar arraythat has a pitch in the range of about 0.1 nm to 10 μm. The LUT 57 ofthe core circuit 10 may comprise 256 address entries, each entry oflength 32 bits.

In one embodiment, soft-wiring in the core circuit 10 is implementedusing address events (e.g., Address-Event Representation (AER)). Firingevent (i.e., spike event) arrival times included in address events maybe deterministic or non-deterministic.

Although certain illustrative embodiments of the invention are describedherein using synapses comprising electronic circuits, the presentinvention is not limited to electronic circuits.

FIG. 1B illustrates a block diagram of an example integrated circuit(“chip circuit”) 100, in accordance with an embodiment of the invention.The chip circuit 100 comprises multiple electronic component units 10deposited on a semiconductor wafer/substrate 50. Each unit 10 is aprocessing element configured for performing arithmetical, logical,and/or input/output (I/O) operations. In one embodiment, each unit 10 isa neurosynaptic neural core circuit 10. In another embodiment, each unit10 is another type of processing element, such as a microprocessor, amicrocontroller, a digital signal processor, a graphics processor, areconfigurable processor, a fixed function unit, a hardware accelerator,a logic gate, etc.

The units 10 are electrically interconnected via a plurality ofconnections 20. The units 10 may be arranged in a one-dimensional gridarrangement, a two-dimensional grid arrangement, a three-dimensionalgrid arrangement, or in a ring or torus topology.

For example, as shown in FIG. 1B, the units 10 may be arranged in atwo-dimensional grid arrangement (e.g., a two-dimensional array) 112.Each unit 10 may be identified by its Cartesian coordinates as unit (i,j), wherein i is a row index and j is a column index of the gridarrangement 112. For example, a unit 10 at row 1 and column 3 of thegrid arrangement 112 is identified as unit (1, 3).

FIG. 2 illustrates an example scan system 200 for a chip circuit 100, inaccordance with an embodiment of the invention. Each unit 10 of a chipcircuit 100 comprises at least one scan chain 110 for initializingand/or testing electronic circuits within the unit 10. For example, ascan chain 110 of a unit 10 may initialize and/or test latches and/ormemory cells of the unit 10. In one embodiment, all units 10 of the chipcircuit 100 have identical scan chains 110 of the same length. Inanother embodiment, different units 10 of the chip circuit 100 havedifferent scan chains 110 (i.e., different scan chain configurations).Therefore, the scan system 200 described in detail later herein may beapplied to a chip circuit 100 comprising multiple units 10 havingidentical scan chains 110 as well as a chip circuit 100 comprisingmultiple units 10 having different scan chains 110.

Each unit 10 further comprises a scan input port 120 for receiving ascan input, a scan clock port 140 for receiving a scan clock signal, anda scan output port 130 for providing a unit scan output. A scan clocksignal received by a unit 10 activates/drives a scan chain 110 of theunit 10 to scan in a scan input via a scan input port 120 of the unit10. A scan input may comprise one or more scan values. If a scan chain110 of a unit 10 is activated, a scan output port 130 of the unit 10scans out a unit scan output indicating whether the initializationand/or testing of the unit 10 is successful.

In one embodiment, the initialization and/or testing of a unit 10 issuccessful if the unit 10 scans out a deterministic/expected unit scanoutput. For example, to test a unit 10, test data (e.g., testvectors/test patterns) are scanned in through a scan chain 110 of theunit 10. The scanned in test data initialize internal circuits of theunit 10. A circuit test is then run on the unit 10. During the circuittest, a scan enable signal for the unit 10 is disabled, the chip circuit10 operates in a normal operation mode, and a clock for the unit 10 istoggled once to drive the internal circuits of the unit 10 for one timestep. Driving the internal circuits of the unit 10 updates the contentsof the scan chain 110 (e.g., updates in a deterministic manner if thereare no defects). The updated contents of the scan chain 110 are thenscanned out as test results. The scanned out test results are comparedagainst expected test results (e.g., derived from a simulation). If thescanned out test results match the expected test results, testing of theunit 10 is successful.

In one embodiment, a unit 10 may have multiple scan chains 110. The scanchains 110 may share the same scan input port 120 and the same scanoutput port 130 of the unit 10. For example, multiplexors may be used tofacilitate the sharing of the scan input port 120 and the scan outputport 130.

In one embodiment, a unit 10 may receive multiple scan clock signals.For example, in one embodiment, a scan system implementinglevel-sensitive scan design (LSSD) utilizes multiple non-overlappingscan clock signals to drive each scan chain 110 of the unit 10.

A scan system 200 may be used to control the scan chains 110 of two ormore units 10 of the chip circuit 100. As described in detail laterherein, the scan system 200 may selectively enable/activate scan chains110 of some units 10, and disable/inactivate scan chains 110 of otherunits 10. Scan input may progress through only the units 10 withenabled/activated scan chains 110, while the current state of otherunits 10 with disabled/inactivated scan chains 110 is preserved. Thescan system may also provide, in parallel, the same scan input to eachunit 10 with an enabled/activated scan chain 110, thereby increasing thespeed at which the chip circuit 100 is initialized/tested.

The scan system 200 comprises at least one scan input component 230. Ascan input component 230 provides, in parallel, identical (i.e., thesame) scan input to two or more units 10 of the chip circuit 100. Forexample, as shown in FIG. 2, a scan input component 230 is connected toa scan input port 120 of multiple units 10, such as Unit 1, Unit 2, Unit3 and Unit 4. The scan input component 230 provides identical scan inputto Unit 1, Unit 2, Unit 3 and Unit 4.

In one embodiment, the scan input component 230 is directly connected toan input pin/pad of chip circuit 100, wherein the input pin/pad receivesand is driven by electrical signals from outside the chip circuit 100.In another embodiment, the scan input component 230 is part of adesign-for-test (DFT) circuit built into the chip circuit 100.

The scan system 200 further comprises a scan clock component 240 forproviding a scan clock signal to two or more units of the chip circuit10. For example, as shown in FIG. 2, a scan clock component 240 isconnected to a scan clock port 140 of multiple units 10, such as Unit 1,Unit 2, Unit 3 and Unit 4.

In one embodiment, the scan system 200 selectively enables/activates ascan chain 110 of a unit 10 using a scan enable signal for the unit 10.Specifically, the scan system 200 further comprises at least one scanselect component 250 and at least one AND unit 210. A scan selectcomponent 250 generates a scan enable signal for a corresponding unit10. In this specification, let scan_en[i] denote a scan enable signalfor unit i of the chip circuit 100, wherein i is a positive integer. Forexample, as shown in FIG. 2, a first scan select component 250 generatesa first scan enable signal scan en[1] for Unit 1, a second scan selectcomponent 250 generates a second scan enable signal scan_en[2] for Unit2, a third scan select component 250 generates a third scan enablesignal scan_en[3] for Unit 3, and a fourth scan select component 250generates a fourth scan enable signal scan_en[4] for Unit 4.

In one embodiment, each scan select component 250 is directly connectedto an input pin/pad of chip circuit 100, wherein the input pin/padreceives and is driven by electrical signals from outside the chipcircuit 100. In another embodiment, each scan select component 250 ispart of a design-for-test (DFT) circuit built into the chip circuit 100.

In one embodiment, a scan select component 250 sets a scan enable signalfor a corresponding unit 10 to either ‘1’ or ‘0’. A scan chain 110 of aunit 10 is enabled/activated when the unit 10 receives both a scan clocksignal and a scan enable signal set to ‘1’. As shown in FIG. 2, for eachunit 10, a scan clock signal is AND'ed with a scan enable signal for theunit 10 using an AND unit 210. The AND of the scan clock signal and thescan enable signal is provided as input to a scan clock port 140 of theunit 10. The scan chain 110 of the unit 10 is enabled/activated if theAND of the scan clock signal and the scan enable signal is ‘1’ (i.e.,the unit 10 has received both a scan clock signal and a scan enablesignal set to ‘1’). The scan chain 110 of the unit 10 isdisabled/inactivated if the AND of the scan clock signal and the scanenable signal is ‘0’ (i.e., the unit 10 has not received a scan clocksignal and/or the scan enable signal for the unit 10 is set to ‘0’). Inanother embodiment, a local clock buffer is used in lieu of an AND unit210. The local clock buffer buffers a scan clock signal from the scanclock component 240, and propagates the buffered scan clock signal asinput to the scan clock port 140 of the unit 10 only when the scanenable signal is set to ‘1’.

The scan system 200 further comprises a chip scan output component 260for maintaining a chip scan output. A chip scan output indicates whetherthe initialization and/or testing of the entire chip circuit 100 issuccessful. The chip scan output is based on unit scan outputs scannedout by units 10 of the chip circuit 100.

The scan system 200 further comprises at least one AND unit 215. Asshown in FIG. 2, for each unit 10, a scan enable signal for the unit 10is AND'ed with unit scan output scanned out of the scan chain 110 of theunit 10 using an AND unit 215. For example, the first scan enable signalscan_en[1] for Unit 1 is AND'ed with unit scan output scanned out of thescan chain 110 of Unit 1 using a first AND unit 215. The second scanenable signal scan_en[2] for Unit 2 is AND'ed with unit scan outputscanned out of the scan chain 110 of Unit 2 using a second AND unit 215.The third scan enable signal scan_en[3] for Unit 3 is AND'ed with unitscan output scanned out of the scan chain 110 of Unit 3 using a thirdAND unit 215. The fourth scan enable signal scan_en[4] for Unit 4 isAND'ed with unit scan output scanned out of the scan chain 110 of Unit 4using a fourth AND unit 215.

In one embodiment, the chip scan output is an exclusive-OR (XOR) of unitscan outputs scanned out by the scan-enabled units 10 of the chipcircuit 100. Specifically, the scan system 200 further comprises an XORtree 265 comprising one or more XOR units (i.e., XOR logic gates) 220for determining the chip scan output. The XOR tree 265 is an examplelogic tree architecture, wherein the XOR units 220 are arranged to formmultiple levels of XOR logic. For example, as shown in FIG. 2, the XORtree 265 comprises a first XOR unit 220 representing a first level ofXOR logic, a second XOR unit 220 representing a second level of XORlogic, and a third XOR unit 220 representing a third level of XOR logic.The first XOR unit 220 determines a first XOR value representing an XORof output from the first AND unit 215 and the second AND unit 215. Thesecond XOR unit 220 determines a second XOR value representing an XOR ofthe first XOR value and output from the third AND unit 215. The thirdXOR unit 220 determines a third XOR value representing an XOR of thesecond XOR value and output from the fourth AND unit 215. If Unit 1,Unit 2, Unit 3 and Unit 4 are the only units 10 of the chip circuit 10,the chip scan output of the chip circuit 10 is equal to the third XORvalue. Therefore, if only the scan enable signals for Unit 1 and Unit 2are set to ‘1’, the chip scan output of the chip circuit 100 is the XORof unit scan outputs scanned out by Unit 1 and Unit 2.

In one embodiment, the chip scan output component 260 is directlyconnected to an output pin/pad of the chip circuit 100, such that thechip scan output is directly sent outside the chip circuit 100. Inanother embodiment, the chip scan output component 260 forwards the chipscan output to a DFT circuit built into the chip circuit 100.

In one embodiment, the scan system 100 has multiple operating modes,such as an individual scan mode, a parallel scan mode, and a delayedscan mode. In the individual scan mode, only one unit 10 of the chipcircuit 100 is scan-enabled (i.e., the scan chain 110 of only one unit10 is enabled/activated). For example, to scan-enable only unit i of thechip circuit 100, the scan chain 110 of unit i is enabled/activated bysetting the scan enable signal for unit i to ‘1’ (i.e., scan_en[i]=1).The scan chains 110 of all other units 10 of the chip circuit 100 aredisabled/inactivated by setting the scan enable signals for all theother units 10 to ‘0’ (i.e., scan_en[j]=0, wherein j !=i). Therefore,the chip scan output of the chip circuit 100 is equal to the unit scanoutput of unit i.

In the parallel scan mode, all units 10 of the chip circuit 100 arescan-enabled (i.e., the scan chain 110 of all units 10 areenabled/activated). To scan-enable all units 10 of the chip circuit 100,all scan enable signals for the units 10 of the chip circuit 100 are setto ‘1’. Further, the scan input component 230 provides, in parallel, thesame scan input to all units 10 of the chip circuit 100. Therefore, thechip scan output of the chip circuit 100 is the parity of all unit scanoutputs scanned out by all units 10 of the chip circuit 100.

In one embodiment, the scan system 200 accelerates scan-basedinitialization of a chip circuit 100. For example, the parallel scanmode may be used to broadcast the same initialization data (e.g.,initialization vectors) to all units 10 of the chip circuit 100. Ifinitializing each unit 10 with different initialization data, theindividual scan mode may be used to scan in unique initialization datainto each unit 10 one at a time.

The parallel scan mode may be used to initialize all units 10 of thechip circuit 100 with the same configuration data. The parallel scanmode may also be used to perform a quick chip test on the chip circuit100 by providing the same test pattern as scan input to all units 10 ofthe chip circuit 100, and analyzing the chip scan output against anexpected chip scan output. If a single unit 10 fails during the chiptest by producing an incorrect unit scan output, the chip scan outputfor the chip circuit 100 is different from the expected chip scanoutput. The chip test fails when the chip scan output for the chipcircuit 100 differs from the expected chip scan output.

While the parallel scan mode may be used to detect a failed chip circuit100, the parallel scan mode does not identify which units 10 of the chipcircuit 100 contributed to the failure of the chip circuit 100 (i.e.,failed units 10). To identify which units 10 of the chip circuit 100 arefailed units 10, individual units 10 of the chip circuit 100 may betested using the individual scan mode. In the alternative, a failed unit10 may be identified using a binary search, wherein half of the units 10receiving the same scan input are activated.

Further, in the parallel scan mode, if two units 10 connected to thesame scan chain system 200 are failed units 10, an exclusive-OR of allunit scan outputs scanned out by units 10 connected to the scan chainsystem 200 may cancel out incorrect unit scan outputs of the two failedunits 10. As a result, the parallel scan mode may fail to detect afailed chip circuit 100.

In the delayed scan mode, all units 10 of the chip circuit 100 arescan-enabled (i.e., the scan chain 110 of all units 10 areenabled/activated), however a scan chain 110 of at least one unit 10 isenabled/activated only after one or more clock delays have elapsed. Inone embodiment, in the delayed scan mode, a scan enable signalscan_en[i] is set to ‘1’ at clock cycle t if i≦t.

For example, when the clock cycle t=1, scan enable signal scan_en[1] forUnit 1 is set to ‘1’, while the scan enable signals scan_en[2] for Unit2, scan_en[3] for Unit 3, and scan_en[4] for Unit 4 are set to ‘0’. OnlyUnit 1 is scan-enabled during the first clock cycle. When the clockcycle t=2, scan enable signals scan_en[1] for Unit 1 and scan_en[2] forUnit 2 are set to ‘1’, while the scan enable signals scan_en[3] for Unit3 and scan_en[4] for Unit 4 are set to ‘0’. Only Unit 1 and Unit 2 arescan-enabled during the second clock cycle. When the clock cycle t=3,scan enable signals scan_en[1] for Unit 1, scan_en[2] for Unit 2 andscan_en[3] for Unit 3 are set to ‘1’, while the scan enable signalscan_en[4] for Unit 4 is set to ‘0’. Only Unit 1, Unit 2 and Unit 3 arescan-enabled during the third clock cycle. When the clock cycle t≧4,scan enable signals scan_en[1] for Unit 1, scan_en[2] for Unit 2,scan_en[3] for Unit 3 and scan_en[4] for Unit 4 are all set to ‘1’. Unit1, Unit 2, Unit 3 and Unit 4 are scan-enabled during the fourth clockcycle and each succeeding clock cycle.

In one embodiment, the parallel scan mode and the delayed scan mode maybe combined to detect a failed chip circuit 100. By combining theparallel scan mode and the delayed scan mode, a failed chip circuit 100may be detected, even when unit scan outputs of two failed units 10cancel each other out. The failed units 10 contributing to the failureof the chip circuit 100 may be identified using triangulation.

FIG. 3 illustrates an example triangulation process to identify a singlefailed unit 10 of a chip circuit 100, in accordance with an embodimentof the invention. Assume a chip circuit 100 comprises n units 10, andeach unit 10 scans in a scan input comprising m scan bits. A singledefect in the entire n*m bits may be detected using triangulation. Forexample, FIG. 3 illustrates using triangulation to identify a singlefailed unit 10 of a chip circuit 100 comprising four units 10 (i.e.,n=4), wherein each unit 10 scans in a scan input comprising three scanbits (i.e., m=3).

Let S_(ij) represent a j^(th) unit scan output of a unit i. Totriangulate a failed unit 10 of the chip circuit 100, parities must bescanned out multiple times in both the parallel scan mode and thedelayed scan mode. Let Sp(i) denote an i^(th) chip scan output of thechip circuit 100 when the chip circuit 100 operates in the parallel scanmode, wherein 1≦i≦m. Let Sd(i) represent an i^(th) chip scan output ofthe chip circuit 100 when the chip circuit 100 operates in the delayedscan mode with one clock delay in between scan start times, wherein1≦i≦m+n−1.

For example, Unit 1, Unit 2, Unit 3 and Unit 4 may be scanned multipletimes in the parallel scan mode followed by the delayed scan mode. Asshown in FIG. 3, a first chip scan output Sp(1) of the chip circuit 100in the parallel scan mode is the parity of S₁₁, S₂₁, S₃₁ and S₄₁. Asecond chip scan output Sp(2) of the chip circuit 100 in the parallelscan mode is the parity of S₁₂, S₂₂, S₃₂ and S₄₂. A third chip scanoutput Sp(3) of the chip circuit 100 in the parallel scan mode is theparity of S₁₃, S₂₃, S₃₃ and S₄₃. A first chip scan output Sd(1) of thechip circuit 100 in the delayed scan mode is S₁₁. A second chip scanoutput Sd(2) of the chip circuit 100 in the delayed scan mode is theparity of S₁₂ and S₂₁. A third chip scan output Sd(3) of the chipcircuit 100 in the delayed scan mode is the parity of S₁₃, S₂₂ and S₃₁.A fourth chip scan output Sd(4) of the chip circuit 100 in the delayedscan mode is the parity of S₂₃, S₃₂ and S₄₁. A fifth chip scan outputSd(5) of the chip circuit 100 in the delayed scan mode is the parity ofS₃₃ and S₄₂. A sixth chip scan output Sd(6) of the chip circuit 100 inthe delayed scan mode is S₄₃.

To triangulate a failed unit 10 of the chip circuit 100, Sp(i) iscompared against an expected chip scan output value to detect a flippedbit, if any. Sd(i) is also compared against an expected chip scan outputvalue to detect a flipped bit, if any. Let Sp(x) denote a flipped bit inscan output in the parallel scan mode, wherein the flipped bit is thex^(th) bit in the scan output. Let Sd(y) denote a flipped bit in scanoutput in the delayed scan mode, wherein the flipped bit is the y^(th)bit in the scan output. The x^(th) bit of the (y−x+1)^(th) unit 10 has adefect bit.

For example, if the unit scan output S₂₃ is flipped (i.e., the thirdunit scan output of Unit 2 has a defect bit), both the third chip scanoutput Sp(3) of the chip circuit 100 in the parallel scan mode and thefourth chip scan output Sd(4) of the chip circuit 100 in the delayedscan mode will be opposite from an expected chip scan output. As x=3 andy=4, the third unit scan output of Unit 2 (i.e., 4−3+1) has a defectbit.

In one embodiment, the delay in scan start times between units 10 in thedelayed scan mode may be longer than one clock cycle (i.e., a multipledelayed scan mode). For example, the scan chain 110 of a unit 10 may beenabled/activated 2, 3 or more clock cycles after the scan chain 110 ofa preceding unit 10 has been enabled/activated. Combining the parallelscan mode and an increasing number of delayed scan modes with differentdelays minimizes the likelihood that a failed scan test goes undetected.

Further, the multiple delayed scan mode facilitates triangulation ofmultiple failed units 10. To triangulate multiple failed units 10 of thechip circuit 100, parities must be scanned out multiple times in boththe parallel scan mode and the multiple delayed scan mode. Let Sp(i)denote an i^(th) chip scan output of the chip circuit 100 when the chipcircuit 100 operates in the parallel scan mode, wherein 1≦i≦m. LetSd_k(i) represent an i^(th) chip scan output of the chip circuit 100when the chip circuit 100 operates in the multiple delayed scan modewith k clock delays in between scan start times, wherein 1≦i≦m+(n−1)*k.The j^(th) bit of unit i contributes to the parity of Sp(j),Sd_1(j+i−1), Sd_2(j+2i−2), Sd_3(j+3i−3) and so on. An n-m algorithm isapplied in the multiple delayed scan mode to detect defect bits.Specifically, a scan test is run to generate n scan outputs Sp, Sd_1,Sd_2, Sd_3, . . . , Sd_n−1. For each bit, n parity bits from the n scanoutputs are identified that cover the bit. If m out of n coveringparities are correct, the bit is correct; otherwise, the bit is a defectbit.

FIG. 4 illustrates an example scan loop system 400 for a unit 10, inaccordance with an embodiment of the invention. The scan loop system 400comprises a multiplexor 420 and a scan loop component 410. Themultiplexor 420 provides the scan chain 110 of the unit 10 with either ascan input (e.g., from a scan input component 230) or a most recent unitscan output generated by the scan chain 110. A scan loop enable signalprovided by the scan loop component 410 controls which value themultiplexor 420 provides to the scan chain 110. The scan loop system 410allows for the unit scan output to loop back into the scan chain 110 andscan out multiple times without losing a transient scan test result.

For example, the scan loop system 410 may be used to detect transienttest failures and locate a failing bit by combining parallel and delayedscan modes. Combining parallel and delayed scan modes requires multiplereads of the same scan test results. The scan loop system 410facilitates multiple reads of the same scan test results.

FIG. 5 illustrates another example scan system 500 for a chip circuit100, in accordance with an embodiment of the invention. In oneembodiment, a chip circuit 100 may comprises multiple units 10 arrangedin a two-dimensional grid arrangement 112, as shown in FIG. 5. Each unit10 may be identified by its Cartesian coordinates as unit (i, j),wherein i is a row index and j is a column index of the grid arrangement112.

A scan chain 110 of a unit 10 is activated based on two scan enablesignals, that is an x-coordinate scan enable signal and a y-coordinatescan enable signal. Specifically, the scan system 500 comprises a firstscan chain component (Y scan chain) 510 for generating y-coordinate scanenable signals. The scan system 500 further comprises a second scanchain component (X scan chain) 520 for generating x-coordinate scanenable signals. In one embodiment, the first scan chain component 510and the second scan chain component 520 at positioned on the left and atthe bottom of the chip circuit 100, respectively.

Each row of the chip circuit 100 has a separate scan input and aseparate scan output. Let si[i] denote the scan input for a row i. Letso[i] denote the scan output for a row i. The rows may be scanned inparallel.

A scan chain 110 for a unit 10 is activated if both an x-coordinate scanenable signal and a y-coordinate scan enable signal for the unit 10 isset to ‘1’. Let scan_en_(x)[j] denote a scan enable signal generated foran j^(th) x-coordinate. Let scan_en_(y)[i] denote a scan enable signalgenerated for an i^(th) y-coordinate. Unit (i,j) is scan enabled whenboth scan_en_(x)[j] and scan_en_(y—)[i] are both enabled (i.e., set to‘1’). For example, as shown in FIG. 5, x-coordinate scan enable signalsare 0, 1, 0 and 1 for a first, a second, a third and a fourthx-coordinate, respectively. Each x-coordinate may be aligned with acolumn. The y-coordinate scan enable signals are 1, 0, 1 and 0 for afirst, a second, a third and a fourth y-coordinate, respectively. Eachy-coordinate may be aligned with a row. As a result, only unit (1,2),unit (1,4), unit (3,2) and unit (3,4) are scan enabled.

FIG. 6 illustrates a flowchart of an example process 400 for scanning achip circuit in a parallel scan mode, in accordance with an embodimentof the invention. In process block 401, scan enable each unit of thechip circuit. In process block 402, scan in an identical test patterninto each unit in parallel. In process block 403, run a circuit test oneach unit based on the scanned in test pattern. In process block 404,scan out test results from each unit in parallel, and compare parityagainst an expected value.

FIG. 7 illustrates a flowchart of an example process 500 fortriangulating a failed unit of a chip circuit using a parallel scan modeand a delayed scan mode, in accordance with an embodiment of theinvention. In process block 501, scan in an identical test pattern intoeach unit of the chip circuit. In process block 502, run a circuit teston each unit of the chip circuit based on the scanned in test pattern.In process block 503, enable scan loop, and scan out test resultsobtained while the chip circuit is operating in a parallel scan mode. Inprocess block 504, scan out test results obtained while the chip circuitis operating in a delayed scan mode. In process block 505, determinewhether both test results are correct (i.e., determine whether the testresults obtained while the chip circuit is operating in the parallelscan mode and the test results obtained while the chip circuit isoperating in the delayed scan mode are expected).

If both test results are correct, proceed to process block 506 whereboth test results indicate that no defects are detected. If at least oneof both test results is incorrect, proceed to process block 507 todetermine whether both test results indicate a single bit failure. Ifboth test results indicate a single bit failure, proceed to processblock 509 where a defect bit in both test results is located bytriangulating a failed bit of a failed unit. If at least one of bothtest result does not indicate a single bit failure, proceed to processblock 508 where a failed unit of the chip circuit is detected byoperating the chip circuit in an individual scan mode or performing abinary search.

FIG. 8 illustrates a flowchart of an example process 600 for locating afailed unit of a chip circuit by combining a binary search with aparallel scan mode and a delayed scan mode, in accordance with anembodiment of the invention. In process block 601, scan-enable units 1through N of a chip circuit, and scan in an identical test pattern intounits 1 through N. In process block 602, run a circuit test on units 1through N of the chip circuit based on the scanned in test pattern. Inprocess block 603, enable scan loop and scan out test results obtainedwhile the chip circuit is operating in a parallel scan mode. In processblock 604, scan out test results obtained while the chip circuit isoperating in a delayed scan mode. In process block 605, determinewhether both test results are correct (i.e., determine whether the testresults obtained while the chip circuit is operating in the parallelscan mode and the test results obtained while the chip circuit isoperating in the delayed scan mode are expected).

If both test results are correct, proceed to process block 606 where thetest results indicate that no defects are detected. If at least one ofboth test results is incorrect, proceed to process block 607 todetermine whether both test results indicate a single bit failure. Ifboth test results indicate a single bit failure, proceed to processblock 609 where a defect bit in both test results is located bytriangulating a failed bit of a failed unit.

If at least one of both test results does not indicate a single bitfailure, proceed to process blocks 608 and 610. In process block 608,the scan is re-run on units 1 through N/2 of the chip circuit. Inprocess block 610, the scan is re-run on units (N/2+1) through N of thechip circuit.

FIG. 9 illustrates a flowchart of an example process 700 for scanningeach unit of a chip circuit using an individual scan mode, in accordancewith an embodiment of the invention. In process block 701, set i to 1.In process block 702, scan enable unit i of a chip circuit, and scan ina test pattern into unit i of the chip circuit. In process block 703,run a circuit test on unit i of the chip circuit. In process block 704,scan out test results for unit i of the chip circuit. In process block705, determine if test results for unit i of the chip circuit iscorrect, and record any defect bits that indicated that unit i is afailed unit.

In process block 706, determine whether all units of the chip circuithave been scanned. If there are remaining units of the chip circuit toscan, proceed to process block 707 where i is incremented by 1. Processblock 707 loops back to process block 702. If all units of the chipcircuit have been scanned, proceed to process block 708 where theprocess 700 ends.

FIG. 10 illustrates a flowchart of an example process 800 fortriangulating one or more failed units of a chip circuit using aparallel scan mode and a multiple delayed scan mode with multipledelays, in accordance with an embodiment of the invention. In processblock 801, scan an identical test pattern into each unit of the chipcircuit. In process block 802, run a circuit test on each unit of thechip circuit based on the scanned in test pattern. In process block 803,enable scan loop and scan out test results obtained while the chipcircuit is operating in a parallel scan mode. In process block 804, scanout test results obtained while the chip circuit is operating inmultiple delayed scan modes with delays ranging from 1 to n−1 clockcycles. In process block 805, locate defect bits that do not contributeto m correct scan out bits. For example, check if every bit in the testresults contributes to the m correct scan out bits. Bits that contributeto more than or equal to m correct scan out bits are marked as correct,whereas bits that contribute of less than m correct scan out bits aremarked as failed.

FIG. 11 is a high level block diagram showing an information processingsystem 300 useful for implementing one embodiment of the presentinvention. The computer system includes one or more processors, such asprocessor 302. The processor 302 is connected to a communicationinfrastructure 304 (e.g., a communications bus, cross-over bar, ornetwork).

The computer system can include a display interface 306 that forwardsgraphics, text, and other data from the communication infrastructure 304(or from a frame buffer not shown) for display on a display unit 308.The computer system also includes a main memory 310, preferably randomaccess memory (RAM), and may also include a secondary memory 312. Thesecondary memory 312 may include, for example, a hard disk drive 314and/or a removable storage drive 316, representing, for example, afloppy disk drive, a magnetic tape drive, or an optical disk drive. Theremovable storage drive 316 reads from and/or writes to a removablestorage unit 318 in a manner well known to those having ordinary skillin the art. Removable storage unit 318 represents, for example, a floppydisk, a compact disc, a magnetic tape, or an optical disk, etc. which isread by and written to by removable storage drive 316. As will beappreciated, the removable storage unit 318 includes a computer readablemedium having stored therein computer software and/or data.

In alternative embodiments, the secondary memory 312 may include othersimilar means for allowing computer programs or other instructions to beloaded into the computer system. Such means may include, for example, aremovable storage unit 320 and an interface 322. Examples of such meansmay include a program package and package interface (such as that foundin video game devices), a removable memory chip (such as an EPROM, orPROM) and associated socket, and other removable storage units 320 andinterfaces 322 which allow software and data to be transferred from theremovable storage unit 320 to the computer system.

The computer system may also include a communication interface 324.Communication interface 324 allows software and data to be transferredbetween the computer system and external devices. Examples ofcommunication interface 324 may include a modem, a network interface(such as an Ethernet card), a communication port, or a PCMCIA slot andcard, etc. Software and data transferred via communication interface 324are in the form of signals which may be, for example, electronic,electromagnetic, optical, or other signals capable of being received bycommunication interface 324. These signals are provided to communicationinterface 324 via a communication path (i.e., channel) 326. Thiscommunication path 326 carries signals and may be implemented using wireor cable, fiber optics, a phone line, a cellular phone link, an RF link,and/or other communication channels.

In this document, the terms “computer program medium,” “computer usablemedium,” and “computer readable medium” are used to generally refer tomedia such as main memory 310 and secondary memory 312, removablestorage drive 316, and a hard disk installed in hard disk drive 314.

Computer programs (also called computer control logic) are stored inmain memory 310 and/or secondary memory 312. Computer programs may alsobe received via communication interface 324. Such computer programs,when run, enable the computer system to perform the features of thepresent invention as discussed herein. In particular, the computerprograms, when run, enable the processor 302 to perform the features ofthe computer system. Accordingly, such computer programs representcontrollers of the computer system.

From the above description, it can be seen that the present inventionprovides a system, computer program product, and method for implementingthe embodiments of the invention. The present invention further providesa non-transitory computer-useable storage medium for initializing andtesting integrated circuits using a scan system that has multipleoperating modes, such as an individual scan mode, a parallel scan mode,and a delayed scan mode. The non-transitory computer-useable storagemedium has a computer-readable program, wherein the program upon beingprocessed on a computer causes the computer to implement the steps ofthe present invention according to the embodiments described herein.References in the claims to an element in the singular is not intendedto mean “one and only” unless explicitly so stated, but rather “one ormore.” All structural and functional equivalents to the elements of theabove-described exemplary embodiment that are currently known or latercome to be known to those of ordinary skill in the art are intended tobe encompassed by the present claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. section 112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or “step for.”

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for triangulating multiple failedprocessing elements, the method comprising: simultaneously scanningmultiple processing elements of a chip circuit in a parallel scan modeand a delayed scan mode; and comparing a parity value representing aparity of all scan outputs scanned out from all processing elementsagainst an expected value to determine whether the chip circuit includesone or more failed processing elements; wherein, in the parallel scanmode, the multiple processing elements are scanned simultaneously and inparallel; and wherein, in the delayed scan mode, at least one processingelement of the multiple processing elements is scanned after one or moreclock delays has elapsed.
 2. The method of claim 1, further comprising:providing a scan enable signal to at least one processing element; andproviding a scan clock signal to at least one processing element;wherein a processing element scans in a scan input and scans out a scanoutput when said processing element receives both a scan clock signaland a scan enable signal that scan enables said processing element. 3.The method of claim 2, further comprising: comparing a scan outputscanned out by a processing element against an expected scan output todetermine whether said processing element is a failed processingelement.
 4. The method of claim 1, wherein each processing element isconfigured to perform at least one of arithmetical, logical andinput/output (I/O) operations.
 5. The method of claim 4, wherein eachprocessing element is a neurosynaptic neural core circuit.
 6. The methodof claim 4, wherein each processing element is one of the following: amicroprocessor, a microcontroller, a digital signal processor, agraphics processor, a reconfigurable processor, a fixed function unit, ahardware accelerator, or a logic gate.
 7. A system comprising a computerprocessor, a computer-readable hardware storage device, and program codeembodied with the computer-readable hardware storage device forexecution by the computer processor to implement a method fortriangulating multiple failed processing elements, the methodcomprising: simultaneously scanning multiple processing elements of achip circuit in a parallel scan mode and a delayed scan mode; andcomparing a parity value representing a parity of all scan outputsscanned out from all processing elements against an expected value todetermine whether the chip circuit includes one or more failedprocessing elements; wherein, in the parallel scan mode, the multipleprocessing elements are scanned simultaneously and in parallel; andwherein, in the delayed scan mode, at least one processing element ofthe multiple processing elements is scanned after one or more clockdelays has elapsed.
 8. The system of claim 7, the method furthercomprising: providing a scan enable signal to at least one processingelement; and providing a scan clock signal to at least one processingelement; wherein a processing element scans in a scan input and scansout a scan output when said processing element receives both a scanclock signal and a scan enable signal that scan enables said processingelement.
 9. The system of claim 8, the method further comprising:comparing a scan output scanned out by a processing element against anexpected scan output to determine whether said processing element is afailed processing element.
 10. The system of claim 7, wherein eachprocessing element is configured to perform at least one ofarithmetical, logical and input/output (I/O) operations.
 11. The systemof claim 10, wherein each processing element is a neurosynaptic neuralcore circuit.
 12. The system of claim 10, wherein each processingelement is one of the following: a microprocessor, a microcontroller, adigital signal processor, a graphics processor, a reconfigurableprocessor, a fixed function unit, a hardware accelerator, or a logicgate.
 13. A computer program product comprising a computer-readablehardware storage device having program code embodied therewith, theprogram code being executable by a computer to implement a method forsimulating slowest and fastest neural dynamics of a neural model, themethod comprising: simultaneously scanning multiple processing elementsof a chip circuit in a parallel scan mode and a delayed scan mode; andcomparing a parity value representing a parity of all scan outputsscanned out from all processing elements against an expected value todetermine whether the chip circuit includes one or more failedprocessing elements; wherein, in the parallel scan mode, the multipleprocessing elements are scanned simultaneously and in parallel; andwherein, in the delayed scan mode, at least one processing element ofthe multiple processing elements is scanned after one or more clockdelays has elapsed.
 14. The computer program product of claim 13,further comprising: providing a scan enable signal to at least oneprocessing element; and providing a scan clock signal to at least oneprocessing element; wherein a processing element scans in a scan inputand scans out a scan output when said processing element receives both ascan clock signal and a scan enable signal that scan enables saidprocessing element.
 15. The computer program product of claim 14,further comprising: comparing a scan output scanned out by a processingelement against an expected scan output to determine whether saidprocessing element is a failed processing element.
 16. The computerprogram product of claim 13, wherein each processing element isconfigured to perform at least one of arithmetical, logical andinput/output (I/O) operations.
 17. The computer program product of claim16, wherein each processing element is a neurosynaptic neural corecircuit.
 18. The computer program product of claim 16, wherein eachprocessing element is one of the following: a microprocessor, amicrocontroller, a digital signal processor, a graphics processor, areconfigurable processor, a fixed function unit, a hardware accelerator,or a logic gate.